Silk fibroin (SF), derived from silkworm Bombyx mori, is a fibrous protein. It is extensively studied in biomedical field, due to inherent and unique biological properties such as biocompatibility, biodegradability, thermo-mechanical properties and minimum inflammatory reaction. SF can be processed in versatile forms such as thin films, sponges, composites, fibers, microspheres, non-woven mats, tubes and hydrogels.
Hydrogels are especially useful for several biomedical applications such as implants, scaffolds in tissue engineering, wound dressings, drug delivery vehicles and in vitro disease models. Hydrogels are a class of highly hydrated polymeric materials and can maintain a distinct 3D porous structure with mechanical and structural integrity similar to characteristics of many natural tissues and extra-cellular matrices. Silk fibroin solutions progressively undergo gelation if stored at physiological pH at room temperature. However, this process happens over a period of few days to months depending on concentration of solution used. This long gelation time of silk fibroin has been a barrier in its applications in the biomedical field (Kaplan et. al. 2012)
Factors affecting the silk fibroin gelation time such as pH, temperature, concentration of solution, accelerating agents have been subject matters of intense research leading to several patents as well as scientific publications. For SF solutions, gelation can also be induced by physical stimuli such as vortexing, sonication and electrical currents. However, these methods are non-physiological. Further, gelation of aqueous solution of SF depends strongly on the pH; it becomes highly unstable at pH near the isoelectric point of fibroin and is converted into macro-porous gels. Whereas, below the isoelectric pH, the gelation time varies between 10-16 hours and slightly above the isoelectric pH the gelation time increases dramatically and gelation occurs over a period of few days to weeks.
Several methods have been proposed and used for accelerating the gelation time of SF. There are reports prevalent in the art that suggest the use of additives such as polyethylene oxide (PEO), Pluronic (Poloxamer) and other polymeric additives as a gelling agent for silk fibroin to attain lower gelation time, under mild conditions. However PEO and Pluronic reduce the gelation time in limited fashion only. Kaplan et al in Acta Biomaterialia 8, 2185-2192 (2012) has proposed the use of Sodium do-decyl sulphate (SDS) for the accelerated gelation of silk. At low surfactant concentrations, hydrophobic interactions among the SF chains play a dominant role in the association, leading to decreased gelation time. At higher concentrations of surfactant, electrostatic repulsive forces among micellar aggregates gradually became dominant and gelation is hindered. These additives are still not ideal additives for forming biologically relevant hydrogels because of their chemical and non-biodegradable nature. Considering the various disadvantages and limitations of the prior art systems, the present inventors developed a novel composition comprising silk fibroin and sophorolipid which has not been reported anywhere before. It was surprisingly found by the present inventors that a biosurfactant like sophorolipid can accelerate the gelation of silk fibroin at physiological and alkaline pH.